Heat oven to 180C/160C. Soak the artichokes in cold water for 20 mins or so to loosen any dirt, then scrub them. Halve the small ones and quarter the bigger ones, and put them in a roasting tin with the split garlic bulb and rosemary. Coat everything with the oil and season. Roast for 45-50 mins until tender inside and crispy outside.
To finish, squeeze the softened garlic cloves from their skins and toss with the roasted artichokes, along with the mace, butter and lemon juice.